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What is Oxidation Number? Hello everyone welcome back today we will learn about Oxidation number explanation from the chapter Redox Reactions.
00:07 Oxidation Number
00:25 Rules of calculating Oxidation Number
02:40 Oxidation State
03:15 Oxidation states of some compounds
04:32 Concepts of Oxidation Number
05:28 Oxidants / Oxidizing Agents
05:40 Reductants / Reducing Agents

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Oxidation number denotes the oxidation state of an element in a compound ascertained according to a set of rules formulated on the basis that an electron pair in a covalent bond belongs entirely to a more electronegative element.


Rules of Calculating Oxidation Number

In elements, in the free or the uncombined state, each atom bears an oxidation number of zero. Evidently each atom in H2, O2, Cl2, O3, P4, S8, Na, Mg, Al has the oxidation number zero.
For ions composed of only one atom, the oxidation number is equal to the charge on the ion. Thus Na+ ion has an oxidation number of +1. N3 has the oxidation number of -3. Hydrogen has an oxidation number of +1. E.g., NaH, LiH, CaH2 contain hydrogens with oxidation numbers of -1. All group seven atoms have oxidation numbers -1 except if the electronegativity is too high. For example, the oxidation number of chloride in HCl is -1 while it is +1 in HOCl. In many compounds oxygen has an oxidation number of -1 while in super oxides, it is -1/2. Group 1A elements usually have an oxidation number of +1 while that of Group 2A elements is +2. In a neutral state, all molecules have an oxidation number of 0. In a polyatomic ion, the algebraic sum of all the oxidation numbers of atoms of the ion must equal the charge on the ion. For example, the oxidation number of S04 ions is -2.
In many compounds, oxygen has an oxidation number of -2.


Oxidation State
A term that is often used interchangeably with the oxidation number is the oxidation state. Thus in CO2, the oxidation state of carbon is +4, that is also its oxidation number. This implies that the oxidation number denotes the oxidation state of an element in a compound. This in turn helps to identify whether the species is present in oxidised form or reduced form.
Concept of Oxidation Number
Oxygen and hydrogen react to form water. The electron exchange here is not very clear.
2H2 (g) + 02 (g) → 2H20 (l)
The H atom as going from a neutral (zero) state in H2 to a positive state in H2O, the O atom goes from a zero state in O2 to a dinegative state in H2O. It is assumed that there is an electron transfer from H to O and consequently H2 is oxidised and O2 is reduced.
H2(s) + Cl2(g) → 2HCl(g)
CH4(g) + 4Cl2(g) → CCl4(l) + 4HCl(g)

Oxidants/ Oxidising Agents
A reagent which can increase the oxidation number of an element in a given substance. These reagents are called as oxidants.
Reductants/ Reducing Agents
A reagent which lowers the oxidation number of an element in a given substance. These reagents are called as reductants.